1. Technical Field
The present invention relates to an engine exhaust purification device, and specifically relates to a technique for suppressing emission of nitrogen oxides (hereafter referred to as “NOx”) and particulates into the atmosphere by a compact structure.
2. Related Art
As a device for purifying engine exhaust by aftertreatment, a selective catalytic reduction (SCR) exhaust purification device that reduces NOx in exhaust by ammonia is known. In this exhaust purification device, a NOx reducing catalyst is placed in an exhaust passage, and a urea water supply device is placed upstream of the NOx reducing catalyst whereas an oxidation catalyst for ammonia purification is placed downstream of the NOx reducing catalyst. Ammonia is generated by hydrolysis of urea added to the exhaust by the supply device, and supplied to the NOx reducing catalyst as a reducing agent. Meanwhile, ammonia (slipped ammonia) that has passed through the NOx reducing catalyst without contributing to NOx reduction is oxidized by the ammonia purification catalyst and as a result purified before being emitted into the atmosphere.
To meet regulations for engine exhaust that will become tighter in the future, a particulate filter may be added to the SCR exhaust purification device. The particulate filter collects diesel particulates especially emitted from a diesel engine by exhaust filtration, thereby reducing diesel particulates. However, if the particulate filter is simply disposed in addition to the NOx reducing catalyst (and the ammonia purification catalyst downstream of the NOx reducing catalyst), an increase in size of a system as a whole is inevitable, causing problems such as a difficulty in vehicle installation and an excessive weight increase.
In view of this problem, a structure in which the NOx reducing catalyst is supported by the particulate filter so that the particulate filter has both a function of collecting particulates and a function of reducing NOx is studied. As such a catalyst supporting particulate filter, Japanese Laid-Open Patent Application Publication No. 2006-183507 discloses, in FIG. 2, a wall-flow particulate filter in which a urea hydrolysis catalyst is supported on an entire inner wall of a filter medium forming an inlet-side cell passage, while a NOx reducing catalyst is supported on an entire inner wall of the filter medium forming an outlet-side cell passage.